A number of attempts have been made to design and fabricate high precision optical sensing systems in which a position sensor is coupled to an optical source and detector using optical fibers. In one known type of system, an encoder is attached to a movable member, and the position of the member is determined by optically interrogating the encoder. The member may be one that rotates, in which case the encoder may comprise a disk that rotates with the member, or may be linearly movable, in which case the encoder moves linearly along with the member.
In an analog system, the encoder typically includes a track that has a continuously variable optical property, such as a continuously variable density or transmission coefficient. Optical energy is transmitted to the encoder by an optical fiber, passes through the variable density track, and is then coupled back to a suitable detector by an optical fiber. Couplers are used to connect the optical fibers to one another, and to other components of the sensor system. The couplers allow each fiber path to be constructed from several discrete lengths of optical fiber. However, such couplers have insertion losses that are neither negligible nor exactly reproducible. Thus the optical attenuation of the fiber-optic link is an unknown and variable factor that must be determined before the position of the encoder can be inferred solely from an end-to-end attenuation measurement. The link attenuation can be measured by transmitting two optical signals having different wavelengths to and from the sensor, and by designing the sensor such that only one signal is attenuated as a function of the encoder position. Such a system may be termed a two wavelength referenced system. Alternatively, the optical modulation technique inside the sensor must be such that the position of the encoder can be found indepently of the fiber link losses.
Attemps have been made to fabricate high-precision, two-wavelength referenced analog encoding systems. However, it has been found that the performance of such systems is limited by the differential loss resulting from the variable stability of the different mode structures of the two optical signals progating along the fibers. The stability of two wavelength referenced systems is generally accepted to be about 1 percent.
In a digital encoding system, the encoder includes a number of parallel coded tracks, each of which represents a specific bit in a binary word. Each track comprises a series of elements, each of which has an optical property that can assume one of two states, such as transmitting or nontransmitting. For each posiition of the digital encoder, the tracks will present a different set of elements, and therefore a different binary word, to the optical interrogation system. The precision is limited only by the highest achievable element density of the least significant track. Wavelength division multiplexing (WDM) is used to interrogate each track with light in a different wavelength range. This arrangement permits optical signals to be coupled to and from the sensor along single fiber-optic cables.
WDM digital encoding systems have been described that use a combination of a GRIN rod lens, a prism, and diffraction grating. Such a system is optically complicated and inefficient, and requires a broad band source for operation. A fundamental problem common to all WDM encoders that use a diffraction grating as the dispersive element is that a reasonably well-collimated beam is required if reasonable resolution and channel width are to be obtained. In particular, one dimension of the beam should be no wider than the dimensions of the elements of the least significant track along the length of the track. This implies that either the encoder element length should be 50-100 times the fiber diameter, or that the beam should be severely masked with a slit. A compromise must therefore be made between resolution, optical efficiency, and the physical size of the sensor. Similar considerations apply to the use of interference filters to demultiplex the channels.